1. Field of the Invention
The present invention relates to a multicomponent system for modifying, degrading or bleaching lignin or lignin-containing materials, and processes for its use.
2. The Prior Art
The primary objective of all processes for the preparation of pulp is the removal of the lignin from the plant starting material. This objective is achieved by using suitable oxidative or reductive chemical process steps which ultimately modify the lignin in a manner in which it can easily be extracted from the pulp product. Traditionally, chlorine or chlorine-containing chemicals, such as hypochlorite or chlorine dioxide, are used in the chemical process steps.
Recently, instead of or in addition to these chlorine-containing compounds, chlorine-free chemicals have been employed. These include, for example, oxygen, hydrogen peroxide, peroxyacids or ozone. Typically, the pulp to be processed is subjected to an extraction step which contains alkali, e.g. NaOH, between the various chemical treatments.
As an additional chlorine-free process step in the chemical processes for the removal of lignin, enzymatic processes are known which should facilitate the chemical removal of the lignin in a multistage delignifier and bleaching sequence. This process step can be carried out as one or more stages in sequential combination with chemical process steps (WO 91/11553). Advantages of this process step include an improvement in the degrees of whiteness which can be achieved. There is also a reduction in the chemical requirement, especially in the amount of chlorine employed.
Using EP 0 487 557 discloses additionally individual enzymes or mixtures of enzymes of related classes in chemical delignification processes using oxygen. These are, for example, enzyme preparations which contain various hemicellulases.
U.S. Pat. No. 5,374,555 discloses a chemical process with additional enzymatic pulp treatment using enzymes from the proteases group. This treatment is carried out as a separate process step which leads to advantages in sequential combination with chemical process steps.
WO 91/11552 describes the use of oxidizing and hydrolytic enzymes in a process for the mechanical production of mechanical pulp. Additives such as ascorbic acid are used to adjust the redox potential to values of &lt;200 mV.
In addition to the use of enzymatic processes combined with chemical processes for the treatment of the pulp, enzymatic processes only used for the treatment of the pulp are also described.
Such processes utilize phenol-oxidizing enzymes. Examples of such enzymes are those from the peroxidases (E.C.1.11.1) and oxidases group (E.C.1.10.3). The enzymes either use hydrogen peroxide or molecular oxygen as an electron acceptor. The use of these oxidative enzymes in bleaching is described (e.g. Paice, M. G., 1995). Manganese peroxidase (MnP), lignin peroxidase (LiP) or laccase (E.C.1.10.3.2), for example, are used. Whereas LiP and MnP typically require hydrogen peroxide as a cosubstrate, laccase works with oxygen as an electron acceptor.
It was still not possible to use peroxidases successfully in bleaching. This is because up to now it has not been possible to solve the problem of peroxidase inactivation by hydrogen peroxide. Also there is no suitable metering technique for hydrogen peroxide in a batchwise industrial process.
The sole use of laccase does not convert the lignin into the desired extractable form. It was possible by addition of suitable mediator substances to laccase to develop an industrial process which can successfully delignify pulp of any type. Such an enzymatic delignifier system, which consists of the components laccase, mediator and oxygen, is described in WO 94/29510.
Further mediators for enzymatic delignifications using laccase are described in WO 7/06244 9.
WO 94/01538 describes the use of cellobiose oxidase in combination with an endoglycanase, for example xylanase and/or oxidoreductase, for example laccase. Moreover, a bleach enhancer is also added in the process. The conditions indicated include an alkaline pH. The efficiency of such a system is very low, since at the pH mentioned the laccase described is largely inactive. Moreover, laccase is inactivated in the presence of hydrogen peroxide. There is also the fact that the compounds mentioned as bleach enhancers show no enhancer activity. This is evident from WO 96/12846, where a use analogous to pulp bleaching, the bleaching of dyed textile fibers, is described:
In Example 1, WO 96/12846 describes the efficiency of different mediators. As is evident from Table 2 in this example, the exemplary compounds mentioned there show no mediator activity.
WO 94/29510 describes the advantageous combination of an enzymatic delignifier system consisting of a laccase and an active mediator. This process is the only known enzymatic process at present which leads to an effective breakdown of lignin. The mediator used here is N--OH-benzotriazole (HBT). In the process described, the addition of further enzymatic components, such as, proteases, is also mentioned.
As shown by oksanen et al. 1997, a combined use of xylanase/laccase/HBT shows only a very slight improvement compared with a xylanase-free use. Advantages resulted only after the xylanase treatment was separated from the laccase/HBT treatment and was carried out as a unique process step.
It is therefore desirable to have available a multicomponent system for modifying, degrading or bleaching lignin and lignin-containing materials. This system achieves better results in delignification than known enzymatic delignification systems and is not affected by the disadvantages of chemical delignification systems.
In addition to the poor performance of the individual enzymatic processes for pulp delignification, the isolation of enzymatic delignification stages on account of the problems described above and the stepwise procedure in the delignification of pulp resulting therefrom is a considerable disadvantage of these processes. Due to the necessity of washing and extraction steps between the different enzyme treatments, the processes cannot be integrated economically into the existing delignification and bleaching sequences.
It is therefore also desirable to have available enzymatic processes for the treatment of lignin and lignin-containing materials which combine several enzymatic process steps in a single process stage.